FIGS. 14(a) to 14(c) show a buffer closing device disclosed in Patent Document 1. The reference alphabet F represents a crossbar structuring a main body frame which is a fixed body. In the crossbar F, there is provided a transverse groove. A sliding door D which is the moving body includes a door neck D1 which fits into the transverse groove thereof. Then, in the structure, there is provided a retraction tool (a device main body) 1 in the sliding door D, and a striker (an engagement member) 2 is attached to the transverse groove of the crossbar F. The striker 2 is attached inside the transverse groove of the crossbar F, and an engagement protrusion 2a protrudes downwardly. On the other hand, the retraction tool 1 includes a case 3; a latch (an engagement member) 5 disposed inside the case 3, and switched between a stand-by position (see FIG. 14(b)) locked to be incapable of sliding, and a retraction position (see FIG. 14(c)) in which the locking is released so as to be capable of sliding; and a coil spring 40 as a closing mechanism B. The reference numeral 12 represents a first cylinder, and the first cylinder 12 drives a first piston 11. The reference numeral 6 represents a piston rod, and the piston rod 6 pivotally supports the latch 5. The case 3 includes a concave groove 3a whose upper side is open up to a middle from one end; and a first guide groove 25 and a second guide groove 26, which extend backward and forward. The concave groove 3a allows the engagement protrusion 2a to come in and out of the case. The guide grooves 25 and 26 guide sliding of the latch 5. The guide groove 25 includes a folded portion 25a wherein one end side of the case is folded downwardly, and which locks the latch 5 to be incapable of sliding. Then, the case 3 forms one portion of the door neck which continues to the door neck D1 in an attachment state wherein a lower side is provided to be buried in a concave-like portion provided in an upper end face of the sliding door D.
In the above-mentioned buffer closing device, as shown in FIG. 14(b), when the latch 5 is in the standby position, an engagement axis 21 provided in the latch 5 is locked in the folded portion 25a, and a position of the latch 5 is controlled although the latch 5 is receiving an urging force accumulated in the coil spring 40. Then, from a state of FIG. 14(b), when the sliding door D is moved in a left direction further, the engagement protrusion 2a hits against a corresponding inner face of the latch 5, and the latch 5 is rotated by the force thereof so as to be switched to the retraction position of FIG. 14(c) from the standby position. In the retraction position, in a state wherein the latch 5 has restrained the engagement protrusion 2a, the engagement axis 21 is unlocked from the folded portion 25a so as to enter into a horizontal groove portion of the guide groove 25. Consequently, the latch 5 is slid by the urging force accumulated in the coil spring 40 so as to automatically switch the sliding door D to the closed position. Also, in an opening operation of the sliding door D from the closed position, when the latch 5 is slid to a right side of the figure, accompanied by that, as shown in FIG. 14(b), the urging force is accumulated. Furthermore, when the sliding door is moved in an open direction, the latch 5 is switched to the standby position again.
FIG. 15(a) and FIG. 15(b) show the movement assist device (the slide assist mechanism) disclosed in Patent Document 2. In the structure, a main body frame which is a fixed body movably holds a sliding door which is a moving body, and in the sliding door, a striker (a projecting body) 9 is provided to protrude on an upper end face. A retraction tool (a retraction unit) comprises a case 1 attached to the main body frame; sliders 2A and 2B slidably disposed in the case 1; latches 5 pivotally supported to be capable of rotating through a shaft 8 relative to each slider; and urging devices 3. Here, the sliders 2A and 2B include convex portions 22 on upper and lower faces. Each convex portion 22 fits into guide grooves 12b and 16b provided on upper and lower faces of the case 1, and is slid while being guided by the upper and lower guide devices. Each latch 5 includes protrusions 52 on upper and lower faces. Each protrusion 52 fits into guide grooves 14 and 19 provided on the upper and lower faces of the case 1, and is slid while being guided by the upper and lower guide devices. The guide grooves 14 and 19 comprise the guide grooves 12b and 16b; parallel straight grooves 14a and 19a; and approximately L-shaped locking grooves 14b and 19b provided on both sides of the straight grooves 14a and 19a. 
In the aforementioned structure, the latches 5 on both sides in FIG. 15(a) and the right latch 5 in FIG. 15(b) are in the standby position. In the standby position, the upper and lower protrusions 52 are locked in the corresponding locking grooves 14b and 19b, and positions of the latches 5 are controlled although the latches 5 are receiving urging forces accumulated in the urging devices 3 together with the slider 2A. Then, from a state of FIG. 15(a), when the sliding door on a left side, which is not shown in the figure and is in the open position, is operated to move in a closing direction from the open position, the striker 9 hits an inner face of a hook portion 50 of the corresponding latch 5, and the latch 5 is rotated by the force thereof so as to be switched to the retraction position as shown on the left side in FIG. 15(b) from the standby position. In the retraction position, in a state wherein the latch 5 has restrained the striker 9 inside the hook portion, the upper and lower protrusions 52 enter into the straight grooves 14a and 19a from the locking grooves 14b and 19b so as to be unlocked. Consequently, the latch 5 and the slider 2A are slid by the urging force accumulated in the urging device 3 so as to automatically switch the sliding door to the closed position through the striker 9. Also, when the striker 9 is slid to the left side in the figure together with the latch 5 by the opening operation of the sliding door from the closed position, accompanied by that, the urging force is accumulated in the urging device 3. Furthermore, when the sliding door is moved in the open direction, the latch 5 is switched to the standby position again.